Thin sheets made of an aluminium-magnesium-scandium alloy for aerospace applications

ABSTRACT

The invention relates to a method for producing a wrought product made of an aluminum alloy composed, in wt %, of Mg: 3.8-4.2; Mn: 0.3-0.8 and preferably 0.5-0.7; Sc: 0.1-0.3; Zn: 0.1-0.4; Ti: 0.01-0.05; Zr: 0.07-0.15; Cr: &lt;0.01; Fe: &lt;0.15; Si&lt;0.1; wherein the homogenization is carried out at a temperature of between 370° C. and 450° C., for between 2 and 50 hours, such that the equivalent time at 400° C. is between 5 and 100 hours, and the hot deformation is carried out at an initial temperature of between 350° C. and 450° C. The invention also relates to hot-worked products obtained by the method according to the invention, in particular sheets with a thickness of less than 12 mm. The products according to the invention are advantageous as they offer a better compromise in terms of mechanical strength, toughness and hot-formability.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.16/342,096, filed 15 Apr. 2019, which is a National Stage entry ofInternational Application No. PCT/FR2017/052856 filed 17 Oct. 2017,which claims priority to French Patent Application No. 1660049, filed 17Oct. 2016, the contents of each of which are hereby incorporated byreference in their entirety.

BACKGROUND Field of the Invention

The invention relates to a method for producing wrought products made ofan aluminum-magnesium alloy, also known as a 5XXX series aluminum alloyaccording to the Aluminium Association, more particularly Al—Mg alloyproducts containing Sc having a high mechanical strength, high toughnessand good formability. The invention further relates to productsobtainable by said method, as well as to the use of these productsintended for transportation and in particular for aircraft andspacecraft construction.

Description of Related Art

Wrought products made of an aluminum alloy are developed in particularto produce structural elements intended for the transportation industryand in particular for the aeronautics industry and the aerospaceindustry. In these industries, product performance must be constantlyimproved and new alloys are developed in particular in order to providea high mechanical strength, low density, high toughness, excellentcorrosion resistance and very good formability. In particular, formingcan take place under heat, for example by creep forming, and themechanical properties must not deteriorate after this forming process.

Al—Mg alloys have been extensively studied in the transportationindustry, in particular that of road and sea transportation, due to theexcellent properties thereof for use in such industries, such as theweldability, corrosion resistance and formability thereof, in particularin low-worked tempers such as the O temper and H111 temper.

However, these alloys have a relatively low mechanical strength for theaeronautics industry and aerospace industry.

U.S. Pat. No. 5,624,632 discloses an alloy composed of 3-7 wt %magnesium, 0.03-0.2 wt % zirconium, 0.2-1.2 wt % manganese, up to 0.15wt % silicon and 0.05-0.5 wt % of an element forming dispersoids in thegroup consisting of scandium, erbium, yttrium, gadolinium, holmium andhafnium.

U.S. Pat. No. 6,695,935 discloses an alloy composed, in wt %, of Mg3.5-6.0, Mn 0.4-1.2, Zn 0.4-1.5, Zr max. 0.25, Cr max. 0.3, Ti max. 0.2,Fe max. 0.5, Si max. 0.5, Cu max. 0.4, and one or more elements in thegroup: Bi 0.005-0.1, Pb 0.005-0.1, Sn 0.01-0.1, Ag 0.01-0.5, Sc0.01-0.5, Li 0.01-0.5, V 0.01-0.3, Ce 0.01-0.3, Y 0.01-0.3, and Ni0.01-0.3. Patent application WO 01/12869 discloses an alloy composed, inwt %, of 1.0-8.0 wt % Mg, 0.05-0.6 wt % Sc, 0.05-0.20 wt % Hf and/or0.05-0.20 wt % Zr, 0.5-2.0 wt % Cu and/or 0.5-2.0 wt % Zn andadditionally 0.1-0.8 wt % Mn.

Patent application WO2007/020041 discloses an alloy composed, in wt %,of Mg 3.5 to 6.0, Mn 0.4 to 1.2, Fe<0.5, Si<0.5, Cu<0.15, Zr<0.5,Cr<0.3, Ti 0.03 to 0.2, Sc<0.5, Zn<1.7, Li<0.5, Ag<0.4, optionally oneor more elements forming dispersoids in the group consisting of erbium,yttrium, hafnium, and vanadium, each <0.5 wt %.

The products described in these patents are not sufficient in terms ofoffering a compromise between mechanical strength, toughness andhot-formability. In particular, it is important that the mechanicalproperties do not deteriorate after heat treatment at 300-350° C., whichis a typical temperature for forming.

There is thus a need for wrought products made of an Al—Mg alloy with alow density and improved properties compared to those of known products,in particular in terms of mechanical strength, toughness andhot-formability. Moreover, such product must be obtainable according toa reliable and cost-effective production process that can be easilyadapted to a conventional production line.

SUMMARY

The invention firstly relates to a method for producing a wroughtproduct made of an aluminum alloy wherein:

-   -   a) a molten metal bath having an aluminum base is produced,        composed, in wt %, of        -   Mg: 3.8-4.2;        -   Mn: 0.3-0.8; preferably 0.5-0.7;        -   Sc: 0.1-0.3;        -   Zn: 0.1-0.4;        -   Ti: 0.01-0.05, preferably 0.015-0.030;        -   Zr: 0.07-0.15, preferably 0.08-0.12;        -   Cr: <0.01;        -   Fe: <0.15;        -   Si<0.1;        -   other elements ≤0.05 each and ≤0.15 combined, the remainder            being aluminum;    -   b) an unwrought product is cast from said metal bath;    -   c) said unwrought product is homogenized at a temperature that        lies in the range 370° C. to 450° C., for a duration that lies        in the range 2 to 50 hours such that the equivalent time at        400° C. lies in the range 5 to 100 hours, the equivalent time        t(eq) at 400° C. being defined by the formula:

${t\left( {eq} \right)} = \frac{\int{{\exp\left( {{- 2}912{2/T}} \right)}dt}}{\exp\left( {{- 2}912{2/T_{ref}}} \right)}$

-   -   -   where T is the current temperature expressed in Kelvin,            which changes over time t (in hours) and Tref is a reference            temperature of 400° C. (673 K), t(eq) being expressed in            hours, the constant Q/R=29122 K being derived from the            activation energy for the diffusion of Zr, Q=242000 J/mol,

    -   d) the unwrought product thus homogenized is hot-worked with an        initial temperature in the range 350° C. to 450° C. and is        optionally cold-worked;

    -   e) a flattening and/or straightening process is optionally        carried out;

    -   f) an annealing process is optionally carried out at a        temperature that lies in the range 300° C. to 350° C.

The invention secondly relates to a wrought product made of an aluminumalloy having the composition, in wt %,

Mg: 3.8-4.2;

Mn: 0.3-0.8, preferably 0.5-0.7;

Sc: 0.1-0.3;

Zn: 0.1-0.4;

Ti: 0.01-0.05, preferably 0.015-0.030;

Zr: 0.07-0.15, preferably 0.08-0.12;

Cr: <0.01;

Fe: <0.15;

Si<0.1;

other elements ≤0.05 each and ≤0.15 combined, the remainder beingaluminum; obtainable by the method according to the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Unless specified otherwise, all of the indications concerning thechemical composition of the alloys are expressed as a percentage byweight based on the total weight of the alloy. By way of example, theexpression 1.4 Cu means that the copper content expressed in wt % ismultiplied by 1.4. The designation of the alloys is provided inaccordance with the regulations of The Aluminium Association, known tothose skilled in the art.

The definitions of the tempers are indicated in European standard EN 515(1993). The tensile static mechanical properties, in other words theultimate tensile strength R_(m), the tensile yield stress at 0.2%elongation R_(p0.2), and the elongation at rupture A %, are determinedby a tensile test according to standard NF EN ISO 6892-1 (2009), wherebythe sampling and the direction of the test are defined by standard EN485-1 (2016). The plane strain toughness is determined by a curve of thestress intensity factor K_(R) as a function of the effective crackgrowth Δa_(eff) known as the R-curve, according to standard ASTM E 561(2010). The critical stress intensity factor K_(C), in other words theintensity factor that makes the crack unstable, is calculated from theR-curve. The stress intensity factor K_(CO) is also calculated byassigning the initial crack length to the critical load, at the start ofmonotonic loading. These two values are calculated for a specimen of therequired form. K_(app) represents the factor K_(CO) corresponding to thespecimen that was used to carry out the R-curve test. K_(eff) representsthe factor K_(C) corresponding to the specimen that was used to carryout the R-curve test. K_(R)60 corresponds to the value of K_(R) for aneffective crack growth Δa_(eff)=60 mm.

Within the scope of the invention, the grain structure of the samples ischaracterized in the plane LxTC at mid-thickness, t/2, and isquantitatively assessed after metallographic etching of the anodicoxidation type under polarized light:

-   -   the term “essentially non-recrystallized” is used when the grain        structure has no or few recrystallized grains, generally less        than 20%, preferably less than 15% and more preferably less than        10% of the grains are recrystallized;    -   the term “recrystallized” is used when the grain structure has a        significant proportion of recrystallized grains, generally more        than 50%, preferably more than 60% and more preferably more than        80% of the grains are recrystallized.

Unless specified otherwise, the definitions of standard EN 12258-1(1998) apply.

Within the scope of the present invention, a “structural element” of amechanical construction means a mechanical part for which the staticand/or dynamic mechanical properties are particularly important to theperformance of the structure and for which a structural calculation isusually prescribed or carried out. These are generally elements whosemalfunction is likely to jeopardize the safety of said construction, ofits users or of other persons. For an aircraft, these structuralelements in particular include the elements that comprise the fuselage(such as the fuselage skin, fuselage stiffeners or stringers, bulkheads,circumferential frames, wings (such as the upper or lower wing skin),stringers or stiffeners, ribs, spars, floor beams and seat tracks) andthe tail unit in particular comprised of horizontal or verticalstabilizers, as well as the doors.

The inventors hereof have observed that, for a composition according tothe invention, an advantageous wrought product can be obtained bycontrolling the homogenization conditions, the mechanical properties ofwhich advantageous wrought product offer a compromise between mechanicalstrength and useful toughness for the aircraft construction industry,and the properties whereof are stable after heat treatment correspondingto hot-forming conditions.

According to the invention, a molten metal bath having an aluminum baseis produced composed, in wt %, of Mg: 3.8-4.2; Mn: 0.3-0.8, preferably0.5-0.7; Sc: 0.1-0.3; Zn: 0.1-0.4; Ti: 0.01-0.05, preferably0.015-0.030; Zr: 0.07-0.15, preferably 0.08-0.12; Cr: <0.01; Fe: <0.15;Si<0.1; other elements ≤0.05 each and ≤0.15 combined, the remainderbeing aluminum.

The composition according to the invention is noteworthy as a result ofthe low quantity of added titanium from 0.01-0.05 and preferentiallyfrom 0.015 to 0.030 wt % and preferably from 0.018 to 0.024 wt % and asa result of the absence of added chromium, the content whereof is lessthan 0.01 wt %. The high static mechanical properties (Rp0.2, Rm) areobtained despite these small additions, as a result of thecarefully-controlled homogenization conditions. Thus, surprisingly,recrystallisation can be prevented during the hot-forming process withlow quantities of added titanium and without added chromium, whilesimultaneously procuring high static mechanical properties, which werein particular possible to obtain by adding high quantities of Cr and Ti,and a high toughness.

Mn, Sc, Zn and Zr must be added in order to obtain the desiredcompromise between the mechanical strength, toughness andhot-formability. The iron content is kept below 0.15 wt %, andpreferably below 0.1 wt %. The silicon content is kept below 0.1 wt %,and preferably below 0.05 wt %. The presence of iron and silicon inexcess of the aforementioned maximum values has a negative impact, inparticular on toughness. The remaining elements are impurities, i.e.elements whose presence is unintentional, the presence whereof must belimited to 0.05% each and to 0.15% combined and preferably to 0.03% eachand to 0.10% combined.

According to the invention, said unwrought product is homogenized at atemperature that lies in the range 370° C. to 450° C., for a durationthat lies in the range 2 to 50 hours such that the equivalent time at400° C. lies in the range 5 to 100 hours,

-   -   the equivalent time t(eq) at 400° C. being defined by the        formula:

${t\left( {eq} \right)} = \frac{\int{{\exp\left( {{- 2}912{2/T}} \right)}dt}}{\exp\left( {{- 2}912{2/T_{ref}}} \right)}$

wherein T is the current temperature expressed in Kelvin, which changesover time t (in hours) and Tref is a reference temperature of 400° C.(673 K), t(eq) being expressed in hours, the constant Q/R=29122 K beingderived from the activation energy for the diffusion of the Zr, Q=242000J/mol.

Preferably, the homogenization duration lies in the range 5 to 30 hours.Advantageously, the equivalent time at 400° C. lies in the range 6 to 30hours.

A too low homogenization temperature and/or a too short homogenizationduration does not allow for the formation of dispersoids to controlrecrystallisation. Surprisingly, when the homogenization temperature istoo high and/or when the homogenization duration is too long, theproperties obtained are unstable at the conventional hot-formingtemperature of 300-350° C., in particular since the productsrecrystallize.

Hot working can be carried out immediately after homogenization withoutcooling to ambient temperature, whereby the initial hot workingtemperature must lie in the range 350 to 450° C. Alternatively, theunwrought product can be cooled to ambient temperature afterhomogenization and then reheated to an initial hot working temperaturethat lies in the range 350 to 450° C. In the case of reheating, theequivalent time at 400° C. during reheating must be kept low, generallyless than 10%, compared to the equivalent time at 400° C. duringhomogenization.

During hot working, the temperature of the metal can, in some cases,rise, however the equivalent time at 400° C. during hot working must bekept low, generally less than 10%, compared to the equivalent time at400° C. during homogenization. In any case, the temperature during hotworking must preferably not exceed 460° C. and preferentially not exceed440° C. After hot working, cold working can be carried out.

In a first embodiment, working is carried out by rolling in order toobtain a sheet metal. In this first embodiment, the final thickness ofthe sheet metal obtained is less than 12 mm.

In a second embodiment, working is carried out by extrusion in order toobtain a profile.

In a first embodiment, hot working is generally carried out until athickness of about 4 mm is reached, then cold working is carried out fora thickness that lies in the range 0.5 to 4 mm.

After the hot- and optionally cold-working process, a flattening and/orstraightening operation can advantageously be carried out. Duringflattening and/or straightening operations, the permanent set isgenerally less than 2%, preferably less than about 1%. An annealingprocess is optionally performed at a temperature that lies in the range300° C. to 350° C. The annealing time generally lies in the range 1 to 4hours. The main purpose of this annealing process is to stabilize themechanical properties such that they are not altered during a subsequentforming process at a similar temperature. The products according to theinvention have the advantage of having very stable mechanical propertiesat this temperature. Thus, for products whose final thickness of 4 to 6mm is obtained by hot rolling, the static mechanical property variationis no greater than 10% and preferably no greater than 6% after annealingbetween 300 and 350° C., and for products whose final thickness of about2 mm is obtained by cold rolling, the static mechanical propertyvariation is no greater than 40% and preferably no greater than 30%after annealing between 300 and 350° C. Within the scope of the methodaccording to the invention, it is thus possible not to perform astabilizing annealing process and to immediately carry out forming, inparticular for products whose final thickness is obtained by hotrolling. Thanks to the method of the invention, the products accordingto the invention retain an essentially non-recrystallized grainstructure after annealing at between 300 and 350° C.

Sheet metal having a thickness of less than 12 mm obtained by the methodof the invention is advantageous, preferably having the followingcharacteristics:

(a) a tensile yield stress measured at 0.2% elongation in the LTdirection of at least 250 MPa, and preferably of at least 260 MPa and/or

(b) a tensile yield stress measured at 0.2% elongation in the Ldirection of at least 260 MPa, and preferably of at least 270 MPa,whereby these properties are achieved even in the case wherein theoptional annealing step at a temperature in the range 300° C. to 350° C.is carried out.

Advantageously, sheet metal having a thickness of less than 4 mmobtained by the method of the invention has a tensile yield stressmeasured at 0.2% elongation in the LT direction of at least 300 MPa, andpreferably of at least 320 MPa, whereby these properties are achievedeven in the case wherein the optional annealing step at a temperature inthe range 300° C. to 350° C. is carried out.

The sheet metal according to the invention preferably has advantageoustoughness properties, in particular:

(c) a toughness K_(R)60, measured on specimens of type CCT760 in the L-Tdirection (where 2ao=253 mm), for an effective crack growth Δa_(eff) of60 mm, of at least 155 MPa √{square root over (m)}, and preferably of atleast 165 MPa √{square root over (m)} and/or

(d) a toughness K_(R)60, measured on specimens of type CCT760 in the T-Ldirection (where 2ao=253 mm), for an effective crack growth Δa_(eff) of60 mm, of at least 160 MPa √{square root over (m)}, and preferably of atleast 170 MPa √{square root over (m)}.

Preferably, for the products according to the invention, the toughnessK_(R) in the T-L direction is greater than that in the L-T direction.

Preferably, the toughness Kapp, measured on specimens of type CCT760 inthe T-L direction (where 2ao=253 mm), is at least 125 MPa, andpreferably at least 130 MPa. The products according to the invention canbe formed at a temperature that lies in the range 300° C. to 350° C. inorder to obtain structural elements for an aeroplane, preferablyfuselage elements.

Aircraft fuselage elements according to the invention are advantageousbecause they have

-   -   (a) a tensile yield stress measured at 0.2% elongation in the LT        direction of at least 250 MPa, and preferably of at least 260        MPa and/or    -   (b) a tensile yield stress measured at 0.2% elongation in the L        direction of at least 260 MPa, and preferably of at least 270        MPa.

EXAMPLES Example 1

A plurality of slabs with a thickness of 400 mm were cast, thecomposition whereof is provided in Table 1.

TABLE 1 Composition in wt % (analyzed by spark optical emissionspectrometry, S-OES). Si Fe Cr Mn Mg Zn Ti Zr Sc A 0.02 0.05 <0.01 0.624.05 0.28 0.023 0.10 0.19 B 0.02 0.04 <0.01 0.59 3.99 0.29 0.038 0.100.19

The slab made of alloy A was homogenized for 5 h at 445° C., whereas theslab made of alloy B was homogenized for 15 h at 515° C. The slabs thushomogenized were hot rolled immediately after homogenization with ahot-rolling starting temperature of 415° C. for slab A and 480° C. forslab B, in order to obtain 4 mm thick sheets.

The tensile static mechanical properties of the sheet made of alloy Aremained high, both in the hot-rolled temper (HR) and in the annealedtemper (annealing treatment for 4 h at 325° C.), whereas those of thesheet made of alloy B deteriorated after annealing.

TABLE 2 Static mechanical properties obtained for the different sheet inthe hot-rolled temper (HR) and in the annealed temper (4 h at 325° C.).Alloy A sheet Alloy B sheet Thickness 4 mm Thickness 4 mm HR AnnealingHR Annealing Rp0.2 L, MPa 303 289 287 233 Rm L, MPa 400 393 364 352 A L,% 14.5 16.2 14.8 17.6 Rp0.2 LT, MPa 311 292 276 238 Rm LT, MPa 396 387361 349 A LT, % 17.7 19.5 18.2 23.0 Kapp MPa✓m L-T 129.9 129.1 128.5Kapp MPa✓m T-L 134.9 134.0 125.8 Kr60 MPa✓m L-T 172.9 171.5 171.2 Kr60MPa✓m T-L 178.9 177.1 164

The 4-mm sheets were cold rolled to a thickness of 2 mm by threepassages, without intermediate heat treatment, then underwentflattening. Different heat treatments were carried out after coldrolling. The tensile mechanical test results are shown in table 3.

TABLE 3 Static mechanical properties obtained for the differentcold-rolled sheets having undergone annealing under differentconditions. Alloy A sheet Alloy B sheet Annealing Thickness 2 mmThickness 2 mm after cold Rp02 Rm A % Rp02 Rm A % rolling (LT) (LT) LT(LT) (LT) LT — 417 466 9.95 358 422 10.5 2 h 275° C. 349.5 415 19 256355 18.2 2 h 325° C. 333 405 21.7 168 311 23.0 2 h 375° C. 297.5 39321.4 156 301 23.1

The grain structure of the sheets was observed after metallographicetching of the anodic oxidation type under polarized light after coldrolling (CR) or after cold rolling and annealing for 2 h at 325° C.

A qualitative assessment of the microstructure was carried out:

Table 4 shows the results of the microstructural observations of thesheets of compositions A and B in the unwrought cold rolling temper andafter annealing treatment (2 h at 325° C.).

TABLE 4 Microstructure (plane LxTC, at mid-thickness) of the sheetsAlloy Reference Microstructure A CR Appreciably non-recrystallized 2 h325° C. Appreciably non-recrystallized B CR Appreciablynon-recrystallized 2 h 325° C. Recrystallized

Alloy A according to the invention has excellent recrystallisationresistance.

Example 2

This example studied the effect that the homogenization conditionsbefore hot working have on the mechanical properties. Blocks made ofalloy A of dimensions 250×180×120 mm were hot rolled under differentconditions until obtaining a thickness of 8 or 12 mm. The conditions aredescribed in Table 5.

TABLE 5 Transformation conditions of the different blocks made of alloyA Initial Homogenization Homogenization T(eq) rolling Final Finalrolling temperature duration at temperature thickness temperature (° C.)(h) 400° C. (° C.) (mm) (° C.) CD2 450 15 298 440 12 329 CD3 400 15 15390 12 319 CD4 450 15 298 440 8 325 CF1 450 5 99 440 8 330 CF2 450 5 9912 327 CF3 400 5 5 405 12 320 CF4 515 17 9341 8 325

The mechanical properties were measured on the sheets having undergonerolling or a treatment. The results are presented in Table 6.

TABLE 6 Static mechanical properties obtained for the different sheetsin the hot rolled temper (HR) and in the annealed temper (4 h at 325°C.). Annealing for 4 h HR at 325° C. Rp0.2 Rm A Rp0.2 Rm A blockdirection MPa MPa % MPa MPa % CD2 L 251 377 15.4 243 370 16.0 CD3 L 286398 14.5 278 391 15.4 CD4 L 260 371 13.6 252 366 16.7 CF1 L 275 381 16.1267 373 17.1 CF2 L 268 390 12.9 262 382 13.8 CF3 L 288 399 14.8 280 39215.4 CF4 L 223 341 15.7 209 339 17.3

The products obtained by the method according to the invention (CD3,CF1, CF2, CF3) have advantageous mechanical properties, in particularRp0.2 in the L direction of at least 260 MPa after hot rolling and afterannealing for 4 h at 325° C.

1. A method for producing a wrought product made of an aluminum alloycomprising: a) producing a molten metal bath having an aluminum base,comprising, in wt %, Mg: 3.8-4.2; Mn: 0.3-0.8; Sc: 0.1-0.3; Zn: 0.1-0.4;Ti: 0.01-0.05; Zr: 0.07-0.15; Cr: <0.01; Fe: <0.15; Si<0.1; otherelements ≤0.03 each and ≤0.10 combined, the remainder being aluminum; b)casting an unwrought product from said metal bath; c) homogenizing saidunwrought product at a temperature that lies in a range of from 370° C.to 450° C., for a duration that lies in a range of from 2 to 50 hourssuch that the equivalent time at 400° C. lies in a range of from 5 to100 hours, the equivalent time t(eq) at 400° C. being defined byformula:${t({eq})} = \frac{\int{{\exp\left( {{- 2}912{2/T}} \right)}dt}}{\exp\left( {{- 2}912{2/T_{ref}}} \right)}$where T is the current temperature expressed in Kelvin, which changesover time t (in hours) and T_(ref) is a reference temperature of 400° C.(673 K), t(eq) being expressed in hours, the constant Q/R=29122 K beingderived from the activation energy for the diffusion of Zr, Q=242000J/mol, d) hot-working the unwrought product thus homogenized with aninitial temperature in a range of from 350° C. to 450° C. and isoptionally cold-worked; e) a flattening and/or straightening process isoptionally carried out; f) an annealing process is carried out at atemperature that lies in a range of from 300° C. to 350° C.
 2. Themethod according to claim 1, wherein the homogenization duration lies ina range of from 5 to 30 hours.
 3. The method according to claim 1,wherein working is carried out by rolling in order to obtain a sheet andwherein a final thickness of the sheet obtained is less than 12 mm. 4.The method according to claim 1, wherein working is carried out byextrusion in order to obtain a profile.
 5. The method according to claim1, wherein at the end of f), forming is carried out at a temperaturethat lies in a range of from 300° C. to 350° C.
 6. The method of claim1, wherein in a), the aluminum base comprises, in wt %, Mg: 3.8-4.2; Mn:0.5-0.7; Sc: 0.1-0.3; Zn: 0.1-0.4; Ti: 0.015-0.030; Zr: 0.08-0.12; Cr:<0.01; Fe: <0.15; Si<0.1; other elements ≤0.03 and ≤0.10 combined, theremainder being aluminum.
 7. The method of claim 1, wherein in c), theequivalent time at 400° C. is in a range of from 6 to 30 hours.